It is known in the prior art to coat metal surfaces with various phenylene oxide polymers to form coatings which provide some degree of corrosion protection to the metal surface.
Certain references suggest the use of electropolymerized polyphenylene oxides in corrosion resistant coatings including the following: M-C. Pham et al., Bull. Societe Chimique France 1985, 1169; M. Vijayan et al., Bull. Electrochemistry 1986, 2, 349; T. F. Otero et al., Makromol. Chem., Macromol. Symp. 1987, 8, 255; and S. Pitchumani et al., J. Electrochem. Soc. India 1990, 39, 211.
One approach that has been described utilizes the electrochemical deposition of various phenylene oxide polymers as coatings onto the metal by utilizing a solution containing the polymerizable phenol monomer(s). See, for example, Chemical Abstracts, Vol. 84, 181695h (1976) and Chemical Abstracts, Vol. 95, 188687t (1981).
An alternative approach to the foregoing electrochemical deposition techniques has relied upon the deposition of the prepolymerized PPO material onto the metal either from solution or as a powder coating:
In U.S. Pat. No. 3,455,736 to H. R. Davis et al., it is taught that certain types of PPO material are deficient in regard to their temperature or solvent resistance (see Col. 1, lines 35-47) and that such disadvantages can be remedied by the use of what is termed xe2x80x9ccertain linear or substantially linearxe2x80x9d PPO materials. Such xe2x80x9cselected polyphenyleneoxide oxide polymerxe2x80x9d (see Col. 3, line 24) would, for example, not have alkyl group substitution on the arylene rings of the polymer and are formed from reaction of potassium p-chlorophenolate; and
More recent U.S. Pat. No. 3,907,613 teaches the use of a xe2x80x9csubstitutedxe2x80x9d PPO material, namely, poly(2,6-dimethyl-1,4-phenylene oxide), to coat metal surfaces, in violation of the teachings of the above described ""736 patent. This patent indicates (at Col. 1, lines 13-21) that PPO films on metal lose their adhesion upon drying and are therefore xe2x80x9cunsuitable to create protective layers on metalxe2x80x9d unless expensive treatments of the metal with a thin layer of copper or silver are performed. As alternative approaches to such an expensive approach, this patent proposes either: (1) that the metal surface be provided with an oxidized conversion surface layer (see Col. 1, lines 28-33), such as by heating the metal (see Col. 1, lines 59-60); (2) that the PPO resin be modified by the inclusion of other resins and/or softening agents (see Col. 1, lines 34-37); (3) that corrosion inhibitor(s) be included in the PPO resin layer (see Col. 1, lines 39-45); or (4) that a further coating of PPO be used on top of the initial layer (see Col. 1, lines 46-49).
The present invention relates to a process in which a metal surface is coated with a substituted polyphenylene oxide polymer, as a neat resin or as a component of a resin mixture, for the purpose of providing corrosion protection to the metal. The coated metal article is heated to enable a chemical reaction between the metal surface and the polyphenylene oxide resin. Heating enhances the polymer""s adhesion to and protection of the metal.
The present invention resides in the use of a substituted PPO coating on a metal article to corrosion protect that article and to the resulting coated metal article. In particular, the heat treatment of such a PPO coating is used to enhance the adhesion of the coating to the metal article and to improve the corrosion protection capability of such a coating.
Any type of metal article which would normally be prone to corrosion when exposed to environmental conditions can be used in the practice of the present invention. Metals which a person of ordinary skill in the art may select for practice of the present invention include carbon steel, stainless steel, aluminum, and other normally corrodible metals.
The xe2x80x9csubstituted PPOxe2x80x9d material which is used to form the corrosion protection layer which is to be placed on the aforementioned article is also well known to persons of ordinary skill in the art and is a commercially available polymer product. The aryl rings contained in the polyphenylene oxide polymer are generally substituted with alkyl groups preferably at the 2- and 6-positions on the ring. Generally speaking, the amount of PPO that is used in the coating to be affixed to the metal article will range anywhere from about 1% by weight to substantially 100% by weight of the coating that is applied to the metal surface. It is within the contemplation of the present invention to admix the PPO polymer with other functional additives or standard coating resins which will remain in the coating. For example, silicon, epoxy resins, polyester resins, polystyrene resins and/or other known softening agents for PPO resins can be used. In addition, corrosion inhibitors or other functional additives can be selected for incorporation into the final PPO-containing coating.
In one embodiment of the present invention, the PPO resin can be dissolved in a suitable hydrocarbon solvent, such as toluene, heptane or hexane, can be applied as a liquid film to the metal article, can be allowed to dry either at ambient or elevated temperature. In another embodiment of the present invention, the PPO resin can be applied to the metal article using powder coating techniques that are well known to the person of ordinary skill in the art.
In accordance with the present invention, a heat treatment of the aforementioned PPO-containing coating is used in order to enhance the adhesion of the coating to the metal article and, possibly, by means of metal-polymer interaction, to thereby improve the corrosion resistance of the coated metal article as compared to an analogous coated metal article that has not been heat treated. In general, the heat treatment can range anywhere from above about room temperature up to about 200xc2x0 C. It is preferred to have the heating to a temperature near the Tg of the PPO resin, which is about 215xc2x0 C.